The present invention relates in general to brush seals and methods of fabricating brush seals and particularly relates to bi-directional brush seals and methods of fabricating wherein accurate control of the bristle cant angle is maintained during manufacture and seal operation and enhanced reliability and improved sealing performance are provided.
Various types of brush seals have been proposed and utilized for various sealing purposes in turbomachinery such as generators and turbines and particularly at the stator-to-rotor interface. Conventional brush seal designs often comprise a bristle pack and a carrier structure for supporting the bristle pack. The bristle pack is an array, preferably in the form of a ring, of densely packed metal bristles or non-metallic (e.g., Kevlar) fibers (hereafter collectively referred to as bristles) projecting from a carrier for engagement with an opposing sealing surface. To facilitate bristle flexibility, e.g., for varying stator-to-rotor clearances and avoidance of excessive metal or fiber stress during rotor excursions, the bristles are arranged at an angle to both the radial and circumferential directions. The angle defined by a radial vector and the bristles is known as the cant angle.
Typically, the carrier structure is a ring or an arc of metal that holds the bristle pack. The carrier often consists of front and back plates for sandwiching the bristle pack between the plates, with the plates being welded along a margin or edge remote from the sealing surface. It will also be appreciated that a gap is typically provided between the front (upstream) plate and the bristle pack.
In brush seal fabrication, it is very difficult to control the bristle cant angle accurately while obtaining a fixed securement of the bristles to the carrier. It will be appreciated that the bristles normally have a very tiny diameter, usually ranging from 0.0028 inches to 0.006 inches and that the bristle pack density ranges from 1200 bristles per inch to 3000 bristles per inch. There is a current lack of techniques for handling the bristles effectively during fabrication of the brush seals and, accordingly, there is a need for an improved method of fabricating a brush seal while maintaining accurate control over the bristle cant angle and an improved brush seal.
In accordance with a preferred embodiment of the present invention, there is provided a brush seal comprised of a single carrier for the bristles whereby a single array of bristles is applied to one surface of the carrier for projecting beyond an edge thereof into engagement with an opposing sealing surface. Preferably, arrays of brush bristles are secured along opposite surfaces of the carrier to project from an edge thereof into engagement with a sealing surface, the two arrays of bristles being spaced from one another at their tips. The bristles are disposed in grooves formed along one or opposite surfaces of a carrier, for example, a ring-shaped carrier. Particularly, grooves are accurately formed in those surfaces at a predetermined angle relative to the edge of the carrier opposite the prospective sealing surfaces. For example, where the carrier is circular about an axis, the grooves are formed at a predetermined cant angle relative to radii from the axis. The grooves are defined by ribs along these surfaces, terminating in tips straddling the grooves. With a plurality of bristles disposed in each groove, the tips of the ribs are deformed to overlie the grooves, retaining the bristles in the grooves with the bristle tips projecting from an edge of the carrier. Alternatively, or conjunctively, an epoxy is disposed along a surface of the carrier, e.g., within the grooves, to facilitate retention of the bristles in the grooves. A bi-directional brush seal is thus provided with bristle packs spaced axially one from the other. This enables sealing between high and low pressure regions on opposite axial sides of the seal is especially useful for cases in which the pressures at the two sides of the seal change periodically and, consequently, the leakage flow may proceed in either direction during operation.
To fabricate the brush seal, and assuming the carrier to be in the form of a ring about an axis, a knurling wheel with oblique cutting ribs on the surface of the wheel is forced into contact with the surface of the bristle carrier ring to form tiny, linearly extending grooves in the surface. It will be appreciated that a pair of such knurling wheels may be provided along opposite sides of the carrier ring to provide the grooves on those opposite surfaces at preferably identical cant angles. The dimensions of the groove are preferably less than a millimeter. It will be appreciated that the angle formed between a groove and a radii of the carrier ring is the same as the desired cant angle. Discrete bristles are then wrapped about the ring, for example, about the edge of the carrier opposite from the sealing surface, and laid into the grooves. The bristles laid in the grooves remain at the desired cant angle and project beyond the edge of the carrier. With the bristles in the grooves, a roller is forced into contact with the carrier surface or a pair of rollers are forced into contact with the opposed surfaces to deform the tips of the grooves to overlie the groove openings, thereby retaining the bristles in the grooves. To further prevent the bristles escaping from the grooves and to protect them during installation and operation, epoxy is provided along the bristle carrier surface(s). Alternatively, the epoxy may be coated along the carrier surface(s) and into the grooves to secure the bristles in the grooves.
It will be appreciated that in conventional brush seals, the bristle-free radial height is defined as the height from the seal xe2x80x9cpinchpoint,xe2x80x9d where bristles are first constrained or pinched within the carrier, to the sealing surface, e.g., the surface of a rotating shaft where the brush seal is arcuate. To adjust the bristle-free radial height in accordance with the present invention, the surface and preferably opposite surfaces of the carrier ring can be recessed, for example, by machining a length of flat steps on the side surfaces of the carrier adjacent the edge from which the bristles project. Thus, free bristle portions extend from the tips of the bristles to the juncture of the bristles and the grooves and lie in spaced relation over the inset or flat steps.
In a preferred embodiment according to the present invention, there is provided a brush seal for sealing between adjacent surfaces comprising a brush seal carrier having an array of grooves spaced from one another along a surface thereof and ribs straddling the grooves, a plurality of bristles disposed in the groove between the ribs and having bristle tips projecting from the grooves beyond an edge of the carrier for sealing against a sealing surface and means carried by the carrier overlying the grooves and the bristles therein for securing the bristles and the carrier to one another.
In a further preferred embodiment according to the present invention, there is provided a brush seal for sealing between adjacent surfaces comprising a brush seal carrier having an array of grooves spaced from one another along a surface thereof and ribs straddling the grooves, the ribs terminating in tips adjacent outer ends thereof, a plurality of bristles disposed in the grooves between the ribs and having bristle tips projecting from the grooves beyond an edge of the carrier for sealing against a sealing surface and the tips of the ribs extending over the grooves to clamp the bristles in the grooves.
In a further preferred embodiment according to the present invention, there is provided a method of forming a brush seal having a carrier and a plurality of bristles, comprising the steps of (a) forming a plurality of grooves along a surface of the carrier, (b) locating the plurality of bristles in the grooves with tips thereof projecting beyond an edge of the carrier and (c) securing the bristles in the groove.